Posted by: GW7AAV | August 11, 2010

Running antenna repairs

Today I had the soldering iron out for some repairs to my portable antennas. Call me last minute dot com if you will, because these repairs have been waiting since my Easter fortnight in the South of England. Helen, Caroline and I were activating hills for Summits on the Air (SOTA). I managed to break both of the linked dipoles I carry and Helen snapped the 2 metre J-pole when the fishing pole mast collapsed in on its self. What makes it worse is we have been out on SOTA since with the jury rigged antennas. Reason for the repairs, SOTA activations as GM7AAV/P with Helen as GM7AAU/P and Caroline airing her new intermediate licence as 2M0YLO/P. We will shortly be in North Scotland for two weeks activating summits.

The J-pole consists of a U shaped stainless steel welding rod and a .75mm diameter length of wire. To keep the U section parallel there are three electrical block connectors. The block connectors are cut to three terminals and the brass removed from the centre terminal. Two of the removed brass terminals are utilised for the feed point by soldering the coax centre and braid to the outside. The terminals can then be adjusted for the lowest SWR. The wire is crimped at one into a straight pin terminal sleeve and is held on to the U shaped stainless steel welding rod by means of the other brass terminal from the ‘chocolate block connector’. A small piece of perspex with two holes is used as a top insulator, about an inch of wire passed through one hole and is held in place with a cable tie. A second cable tie allows the top to be slipped over a fishing pole at the desired height. The U section is held to the mast with a Velcro strap. To stop the feed point connections breaking the coax feeder is cable tied to the bottom of the U. The repair to this antenna consisted of fitting a new wire and straightening the U section. Bearing in mind this antenna has been being used for five years it has held up well. It has worked well and I was very impressed at the signals I was getting when I tested it this afternoon in my garden.

The linked dipoles use commercial dipole centres and the wires had broken at the place were the cables were cable tied to take the strain off the where the eye terminals are soldered to the wire. I replaced the 10m band sections on both antennas and was able to use some .75mm diameter hook up wire similar to what I used when I made them, but with insulation twice as thick. Hopefully that will make them last a bit longer, but the older of these two antennas is five years and the newer one four years, which bearing in mind the times the wind has broken fishing poles is not too bad.

Some of you may be wondering what a linked dipole is and I suggest you look at these pages from my friend John GW4BVE. ONE / TWO /THREE /FOUR /FIVE Basically a dipole is cut for the highest band you want, in this case 10m. The ends of the dipole are terminated in a connector, such as in John’s and my second version Anderson Power Poles or on my first version automotive bullet connectors. The latest trend amongst SOTA activators is for using the gold plated ‘banana’ bullet connectors favoured by the radio controlled model fraternity, which are cheaply available on eBay. An insulator made from perspex or some other insulating material is cable tied about an inch and a half from the connector and then the next section is added on the other end of the insulator and so on. You can add as many links/bands as you need. No tuner is needed if you make each section carefully enough. It is more efficent if you tune (trim) the sections for the exact frequencies you intend to operate. Mine are tuned to 3.666, 5.3985, 7.118, 14.285, 28.495 MHz but I can cover the whole of each band with the only a slight change in swr up to the top end of 80m where it approaches 2-1. The linked dipole is unbeatable as a portable antenna as there is no need to carry a heavy tuner and there are none of the losses associated with the use of the mis-named device, which should really be called a matching unit.

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